In a conventional production of high-strength plated steel wire for PWS, hot-rolled wire rods can be subjected to a patenting treatment as necessary, and can then be drawn out to form steel wires having a predetermined diameter, and subsequently galvanized to impart corrosion resistance. This series of treatments may be required, in conventional methods, to generate a strength of TS≧2192−61×d (wherein, TS represents the tensile strength (MPa) and d represents the wire diameter (mm)), and possibly ensure satisfactory ductility performance, which can be typically evaluated by the reduction in area at breakage.
In order to satisfy the above requirements, attempts have been made to improve the drawing workability of high carbon wire rods, either by controlling segregations or microstructures within the rod material, or by including a specific element within the rod material.
A reduction in area for patented wired rods can depend on the grain size of austenite, and the reduction in area may be improved by reducing the grain size of the austenite. Accordingly, attempts have been made to reduce the austenite grain size by using carbides or nitrides of Nb, Ti or B or the like as pinning particles.
For example, a wire rod has been proposed in which one or more elements selected from the group consisting of 0.01 to 0.1% by weight of Nb, 0.05 to 0.1% by weight of Zr, and 0.02 to 0.5% by weight of Mo are added as constituent elements to a high carbon wire rod, as described in Japanese Patent No. 2,609,387.
Furthermore, a wire rod in which the austenite grain size can be reduced by adding NbC to a high carbon wire rod has also been proposed, as described in Japanese Unexamined Patent Application, First Publication No. 2001-131697.
In the case of the wire rod described Japanese Patent No. 2,609,387, the constituent elements described herein above can be added to produce a composition that yields increased ductility for the steel wire. However, in the wire rod described in Japanese Patent No. 2,609,387, because each of the added constituent elements is likely expensive, the production costs tend to increase.
In the wire rod described in Japanese Unexamined Patent Application, First Publication No. 2001-131697, the drawing workability can be improved by adding NbC as pinning particles. However, in the wire rod described in Japanese Unexamined Patent Application, First Publication No. 2001-131697, because each of the added constituent elements may be expensive, the production costs tend to increase. Furthermore, Nb may form coarse carbides or nitrides, and Ti may form coarse oxides, and such compounds may act as the origins of breakages, likely causing a deterioration in the drawability.
Increasing the quantities of C and Si within the wire rod components can be one the most economical and effective ways of increasing the strength of high carbon steel wire. However, as the Si content is increased, ferrite precipitation is likely accelerated, and cementite precipitation is suppressed. As a result, even in the case of a steel having a hypereutectoid composition with a C content that exceeds 0.8%, when the steel is cooled from the austenite region during the patenting treatment, proeutectoid ferrites tend to precipitate in the form of platelets along the austenite grain boundaries.
Moreover, because addition of Si likely causes an increase in the pearlite eutectic temperature, a supercooled composition (such as degenerate pearlite or bainite) tends to be generated within the temperature range of 480 to 650° C. that can be typically employed during patenting. As a result, the reduction in area at breakage of the wire rod after patenting treatment tends to decrease, the ductility tends to deteriorate, and the frequency of wire breakages during the drawing process tends to increase, likely causing a reduction in the productivity and yield.
Accordingly, there may be a need to address and/or overcome at least some of the deficiencies described herein above.